Presentation on theme: "Community Ecology The study of how abundance, diversity, and distributions of species vary due to interactions with other organisms and variation in abiotic."— Presentation transcript:
Community Ecology The study of how abundance, diversity, and distributions of species vary due to interactions with other organisms and variation in abiotic conditions
A traveler should be a botanist, for in all views plants form the chief embellishment Charles Darwin, The Voyage of the Beagle Belize
Bird species richness in the Western Hemisphere (Hawkins et al. 2006)
Table Rock, Oregon
Food web of Bear Island: Summerhayes and Elton 1923, Journal of Ecology, 11,
Questions in Community Ecology Why are some species more common than others? What causes spatial and temporal patterns of community change? What are the consequences of diversity patterns for community processes? Can we explain patterns of organization by invoking some universal laws?
A cautionary tale: body size Hutchinsons Homage to Santa Rosalia (1959)posited that coexisting animals (at the same level in a food web) tend to differ in size by a ratio of roughly 1.3 (hypothetically due to character displacement after competition). With thanks to Peter Morin
Largest species (puncata) can occur with others (affinis and macrocephala), but two small spp shouldnt coexistdue to competitive exclusion.
Horn and May (1977) showed that the 1.3 rule also held for non-biological communities
Simberloff and Boecklen (1981) suggested true statistical tests needed (the beginning of the null model movement), and found many reported 1.3 ratios not significant
In some cases, depending on the model, body sizes WERE found to be overdispersed Sonoran Desert Rodent Hutchinsonian ratios Brown 1975 Bowers and Brown 1982
Approaches Community ecology often asks: how do interactions lead to the pattern of organisms/traits? – Experiments, models, observation/inference, often small spatial scales Macroecology asks: what can we infer about processes from the distribution of organisms/traits? – Statistical patterns, usually large spatial or temporal scales
Figure 1.9. The species composition of a local community at any time is a consequence of many factors interacting in a hierarchical fashion. The composition of the species pool of potential community members depends on past evolutionary and historical events, as well as physiological constraints. Dispersal ability and habitat selection influence which members of the species pool arrive in a particular location. Interspecific interactions among those species that manage to arrive in a particular place further inhibit or facilitate the inclusion of species in the community. Morin 1999
Course Topics Competition Predation (incl. trophic cascades) Mutualisms/indirect effects Macroecology Diversity (and neutral theory) Metabolic theory Community Assembly
Direct effects are indicated by solid lines, with signs corresponding to the signs of interactions between the species. Net indirect effects are indicated by broken lines. Morin 1999 Types of interactions
Interaction Strength Interaction strength (IS) often measured by species deletions, with varying response metrics – experiment length important (time for interactions to manifest), as are density and species composition. – Details on metrics in Laska and Wooton, 1998, and Berlow et al, 1999; more recent review in Wooton and Emerson, 2005
Interaction Strength IS as per-capita impact (short term impact of one individual on an individual of another species) There are also several modelling approaches; one popular group is perturbation models (impact of one individual on a population of another species)
Empirical IS indices (Berlow et al)
How can interaction strength be measured? Or, a short primer on how to do community ecology
Field experiments 1 – Paine 1992; removed herbivores from intertidal, measured impact on algae – (E-C)/CM – Few strong negative impacts of consumers; positive impacts due to indirect interactions Sea urchin Chiton Limpets and chitons E = Experimental C = Control M = density of manipulated species
Field Observations – Wooton 1997; uses methods of dynamic food web models to measure per capita consumption and assimilation in birds – Estimated population sizes, consumption rates, took into account time spent searching or sleeping vs. feeding, and time at low tide (when intertidal species available)observational impact similar to estimates from experiments
Wooton analyzed feeding patterns of 17 species of birds, same island as Paine
Lab experiments 1 – Morin et al 1988 measured interaction strength for competitors in experimental ponds: frogs and aquatic insects
Lab experiments 2 – Schmitz 1997; studied old field food webs (grasshoppers, plants, nitrogen)using field and lab experiments – Lab experiments manipulated top-down (herbivore) and bottom-up (nitrogen) factors separately for each plant species; compared with field experiment manipulating both simultaneously for the community.
Inference based on species traits (e.g. relative body sizes of predators and their prey) Wooton Emerson 2005
Pros and cons of different methods Removals assume equilibrium, do not always effectively estimate indirect effects, and Paines index does not work well for rare species otherwise is practical Dynamic index (Wootton) best when community is far from equilibrium (better for comparing short-term results) or when both positive and negative effects occur Observational approaches good when experimentation is not feasible, but require validation
Interaction-strength distributions Usually many weak and few strong
Abiotic conditions and interaction strength Menge et al 1996: compared predation on mussels in wave exposed vs protected
Lawton 1999: Community ecology is a mess The rules are contingent in so many ways….as to make the search for patterns unworkable
Vellend 2010 Selection: processes determining relative success (competition, predation, disease) Drift: changes in abundance to due random effects Speciation: generation of diversity in regional species pools Dispersal: movement in and out of local communities
To Read for next time: Two Papers from TREE, posted on web page, for a discussion of format (mostly) but ideas in the paper are cool too!
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